In order to enhance functionality and input/output (I/O) count, package-on-package (PoP) structures were invented, so that multiple dies may be stacked and interconnected as an integrated packaging assembly. Even though PoP structures allow for space saving, there were still some barriers that need to be overcome.
FIG. 1 illustrates a conventional PoP structure, which includes package substrate 100, and die 102 bonded onto package substrate 100. Package substrate 104 is also bonded to package substrate 100 through solder bumps 110. Further, package substrate 104 has dies 106 bonded thereon. Package substrate 100 may be bonded on a printed circuit board (PCB, not shown).
In the PoP structure as shown in FIG. 1, due to the existence of die 102 and the possible molding compound 112, the distance between package substrates 100 and 104 is increased. As a result, the height of solder bumps 110 is increased, which further causes the increase in the lateral dimensions of solder bumps 110. This results in several problems. With the big solder bumps 110, the count of solder bumps 110 that may be accommodated in the PoP structure is low. To incorporate more solder bumps 110 in order to meet the packaging requirements, the distance between neighboring solder bumps 110 needs to be kept minimal. However, this incurs the risk of bridging between solder bumps 110. Various approaches have been explored to solve these problems, including, for example, forming elongated solder bumps 110 whose heights are significantly greater than their lateral dimensions. However, this posts a demanding requirement to the accuracy of the manufacturing process, and the respective manufacturing process is complicated. In addition, the risk of bridging still exists.